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Bureau of Mines Report of Investigations/1986 



Evaluation of Bearing Plates Installed 
on Full-Column Resin-Grouted Bolts 

By Stephen C. Tadolini and Bryan F. Ulrich 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Report of Investigations 9044 

Evaluation of Bearing Plates Installed 
on Full-Column Resin-Grouted Bolts 

By Stephen C. Tadolini and Bryan F. Ulrich 




UNITED STATES DEPARTMENT OF THE INTERIOR 

Donald Paul Hodel, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 






Ol, 




Library of Congress Cataloging in Publication Data: 



Tadolini, Stephen C 

Evaluation of bearing plates installed on full-column resin-grouted 
bolts. 

(Report of investigations; 9044) 

Bibliography: p. 12. 

Supt. of Docs, no.: I 28.23:9044. 

1. Mine roof bolts, Resin. 2. Coal mines and mining -Colorado. 3. Plates (Engineering), I. 
Ulrich, Bryan F. II. Title. III. Series: Report of investigations (United States. Bureau of 
Mines); 9044. 



TN23.U43 



[TN289.3] 



622 s [622'.28] 



86-600185 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Acknowledgments 2 

General considerations 2 

Instrumentation 4 

Field investigation — Roadside Mine 6 

Field investigation — Bear No. 3 Mine 8 

Conclusions 12 

References 12 

ILLUSTRATIONS 

1. Approximate location of the Roadside and Bear No. 3 Mines 3 

2. Generalized stratigraphic column of the Roadside Mine 3 

3. Generalized stratigraphic column of the Bear No. 3 Mine 4 

4. Compression pad diagram '. 4 

5. Hydraulic U-cell plates and bladder 5 

6. Vertical displacement gauge 6 

7. Roadside Mine test site location 6 

8. Roadside Mine load contours 3 days and 72 days after test site 

installation 7 

9. Roof spalling in Roadside Mine test site 8 

10. Final Roadside Mine load contours 205 days after test site 

installation 9 

11. Bear No. 3 Mine test site location 9 

12. Instrumentation layout in the Bear No. 3 Mine 10 

13. Bear No. 3 Mine load and roof displacement contours 128 days and 

301 days after test site installation 11 





UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 


ft 


foot in inch 


ft 2 


square foot lb pound 


ft' lbf 


foot-pound (force) psi pound per square inch 



EVALUATION OF BEARING PLATES INSTALLED 
ON FULL-COLUMN RESIN-GROUTED BOLTS 



By Stephen C. Tadolini 1 and Bryan F. Ulrich 1 



ABSTRACT 

The Bureau of Mines conducted field investigations in two underground 
mines to determine the actual loads to which bearing plates i were sub- 
jected when installed in conjunction with full-column resin-grouted 
bolts and the roof movements generated by the applied loads. Measured 
loads indicate that the bearing plate is an integral part of the support 
system. Vertical displacement gauges installed to monitor roof dis- 
placements in the test sites show that the highest degrees of loading 
occur in conjunction with the largest amounts of movement. 



Mining engineer, Denver Research Center, Bureau of Mines, Denver, CO. 



INTRODUCTION 



Resin bolting systems continue to gain 
popularity in underground mines through- 
out the United States, and their general 
success under a wide range of geological 
and operational conditions is well docu- 
mented. However, many questions dealing 
with basic support mechanisms and the in- 
fluence of various factors on effective- 
ness in situ remain only partly answered. 
The Bureau of Mines conducted a major re- 
search project to investigate the useful- 
ness of bearing plates installed at the 
bottom (collar) of a full-column, resin- 
grouted bolt. 

The use of full-column, resin-grouted 
bolts to stabilize underground mine roofs 
necessitates the ability to determine 
support characteristics and behavioral 
patterns. These investigations have 
taken two forms in recent research. The 
first form involves investigating support 
systems and individual bolts by an exact 
theoretical solution, or modeling. These 
methods analyze the state of stress 
in and around the bolts in three phases. 
The first phase analyzes the initial 
loading of the bolt-grout-rock. The sec- 
ond phase considers subsequent move- 
ments along the bolt and in the immediate 
area. The third and final phase involves 
the analysis of discontinuous rock move- 
ments along normal bedding planes. The 
results from these types of investiga- 
tions generally conclude that the load 
transfer mechanism in grouted bolts 



dictates that all movements, and thus 
generated loads, are controlled along 
the bolt axis and interbed slips. This 
implies that the bearing plates' effec- 
tiveness is limited to helping retain 
loose material at the mine roof (1-3). 2 

The second form of solution involves 
field investigations designed to analyze 
actual phenomena witnessed in underground 
mines. Bearing plates and, consequently, 
bolts appear to be subjected to large 
amounts of load, which cause plates to 
bend, bolts to elongate, and bolt ends to 
fail completely. When bearing plates at 
the head of a full-column, untensioned, 
properly installed grouted bolt are sub- 
jected to significant loads, they can be 
considered to be contributing to the sup- 
port of the mine roof. Therefore, the 
assumption has been made that load on the 
bearing plate indicates that the plate 
may be an important part of the support 
system. To verify this assumption, full- 
column resin-grouted bolts were installed 
in coal mine roofs equipped with devices 
to measure the load applied during in- 
stallation and subsequent roof loading 
carried by the bearing plates. To deter- 
mine if the loads are related to the 
underground stability, roof deflection 
measurements were recorded. The loads 
were then compared with roof movements 
to determine if a correlation exists 
between the two parameters. 



ACKNOWLEDGMENTS 



The authors would like to thank the 
mine operators and all who contributed to 
the success of this effort. Special 
thanks are given to Jim Diamanti, mine 



manager, Powderhorn Coal Co. , and to Bill 
Bear, president, Bear Coal Co., for their 
continued support and the use of their 
personnel and equipment in this study. 



GENERAL CONSIDERATIONS 



The study includes field test results 
from two mines: the Roadside Mine and 
the Bear No. 3 Mine (fig. 1). Both test 
sites were chosen because of thick coal 
seams in which the mines are located and 
the geological characteristics of the 
rock above the coal seam. The first 
phase of this investigation was conducted 



in the Roadside Mine, owned and operated 
by the Powderhorn Coal Co. The Roadside 
Mine is located in the Book Cliffs Coal- 
field of the Uinta coal region. The 

^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 




FIGURE 1.- Approximate location of the Roadside and Bear 

No. 3 Mines. 



Q) O O 

c c t 

3 ° o 

x o £ 



2 E 

° ,o 

a. ^ 

"S "O 

g .0) 



CP c 



Massive 
sandstone 



Carbonero coal zone 15 -32 ft 

Cameo coal zone 2 - 18 ft 
Rollins Sandstone 



Sandstone and 
shale 



Moncos tongue 

Cozzette coal zone-thickness varies 
Cozzette Sandstone 



Corcoran coal zone -thickness varies 
Corcoran Sandstone 



Palisade coal zone 



Upper Sego Sandstone 
Anchor coal zone 
Lower Sego Sandstone 



Sandstone 



Shale 



FIGURE 2.-Generallzed stratlgraphic column of the Road- 
side Mine. 



geology consists of interf ingering sand- 
stones and shales of Upper Cretaceous age 
with several important coal zones. Bed- 
ding generally dips about 5° northeast 
away from the Uncorapaghre Uplift toward 
the southwest rim of the Piceance Creek 
Basin. A generalized stratigraphic col- 
umn of the Roadside Mine area is shown in 
figure 2. 

The oldest exposed rock, unit in the 
area is the Mancos Shale, a black to 
dark-gray soft shale with occasional thin 
sandstone beds. The Mancos grades upward 
into the Sego Sandstone, a fine-grained, 
buff to light-gray sandstone with some 
gray shale. The Sego is divided into an 
upper and lower member by the Anchor coal 
zone, a tongue of the Mancos Shale, which 
has been mined in some parts of the area. 

The Mount Garfield Formation consists 
of buff and gray, medium-fine-grained 
sandstone interbedded with gray shale. 
There are five economically important 
coal zones in the area. The Palisade 
zone forms the base of the Mount Garfield 



Formation. The Corcoran coal zone and 
the Cozzette coal zone overlie the Cor- 
coran and Cozzette Sandstone Members, re- 
spectively. The Cameo coal zone overlies 
the Rollins Sandstone Member. This zone 
produces most of the coal from the Book 
Cliffs Coalfield. The Roadside Mine pro- 
duces from the Cameo B seam, which is 4.4 
to 9.4 ft thick. The uppermost coal zone 
is the Carbonera. 

The uppermost unit in the area is the 
Hunter Canyon Formation. The Hunter Can- 
yon is a medium-coarse-grained, buff and 
gray, massive cliff-forming sandstone 
with small beds of gray to greenish-gray 
shale. There are no coal deposits in 
this formation (4_"5)« 

The second phase of the investigation 
was performed in the Bear No. 3 Mine, 
owned and operated by the Bear Coal Co. 
The mine is located in the Somerset coal- 
field near Somerset, CO. The oldest ex- 
posed rock unit in the Somerset area is 
the Upper Cretaceous Mancos Shale (fig. 
3). This unit consists of 2,000 to 3,000 



ft of black or dark-gray soft shale with 
the thin sandstone beds. Overlying Man- 
cos is the Mesaverde Formation, also 
of Upper Cretaceous age, here composed 
of four members. The basal member, the 
Rollins Sandstone, is a 150- to 200-ft- 
thick, massive, cliff-forming, white to 
light-yellow-brown sandstone. The lower 
coal (Bowie Shale) member is an inter- 
bedded and lenticular sandstone, silt- 
stone, and shale sequence 250 to 300 ft 
thick; it contains three important coal 
seams. The A seam forms the base of the 
member and is to 5 ft thick. The B 
seam was mined previously directly below 
the Bear No. 3 Mine in the Edwards Mine. 
The C seam, 7 to 9 ft thick, is the seam 
presently being mined. The seams are all 
separated by 33 to 40 ft. 



The upper coal (Paonia Shale) member 
is lithologically very similar to the 
lower coal member but is more lenticular. 
Up to 400 ft thick in some areas, this 
contains two major coal seams, the D and 
E seams. A similar but non-coal-bearing 
member, the Barren Member, overlies the 
upper coal member, bringing the entire 
mine cover to approximately 1,000 ft. 

There are numerous igneous intrusives 
of post-Eocene age throughout the area, 
some of which appear in mine. The coal- 
beds dip north and northeast at 0° to 
6°. Faults and other fractures occur 
throughout the area with stratigraphic 
displacements of 2 to 20 ft (6). 



INSTRUMENTATION 



Several types of instrumentation were 
used in this investigation to measure 
plate loading and roof movements. Com- 
pression pads and hydraulic U-cells were 
used to directly measure the loads 
applied to bearing plates after bolt 



800 
(Approi) 



O =■£ 



3 & 



|| 

1 s 



140- 
200' 






150- 
200' 



2,000- 
3,000' 



Interbedded sandstone, 
siltstone, and shale 



E seam 4-8 ft 
D seam 0- 7 ft 



Interbedded sandstone, 
siltstone, shale, and 
coal 



Interbedded sandstone, 
siltstone, shale, and 
C seam 7 -9 ft co q| 

B seam 10-17 ft 
A seam 0-5 ft 



Massive, cliff-forming 
sandstone 



Shale and sandstone 



FIGURE 3. -Generalized stratigraphic column of the Bear 
No. 3 Mine. 



installation. Vertical displacement 
gauges were installed to measure differ- 
ential roof displacements during test 
site monitoring and support operations. 
In addition, observation holes were 
drilled throughout the test areas to mon- 
itor with a stratascope the locations and 
widths of roof separations. 

Each compression pad (fig. 4) consists 
of a rubber membrane placed between two 
steel plates. The compression pads have 
a working load limit of 32,000 lb with a 
calculated accuracy of ±200 lb. Readings 
of the compression pads are monitored 
with a special calibrated ring that 
measures the change in circumference of 
the rubber membrane as it loads and un- 
loads. Laboratory tests on the ring in- 
dicate that when loads exceed 30,000 lb, 
the accuracy drops to ±1000 lb. The 



8.00" diom- 




FIGURE 4. -Compression pad diagram. 



nature of the pad is to act as a spring 
between the bolthead and the roof. In 
laboratory investigations the pad, after 
being subjected to high loads, failed to 
rebound to its specified unloaded circum- 
ference. The rubber tends to permanently 
deform after continued loading. In this 
investigation only positive loading was 
recorded, eliminating the possibility of 
inaccurate readings. 

The hydraulic U-cells are U-shaped, 
fluid-filled, flat jack-type load cells 
used to measure relative loads between 
the bearing plate and the installed roof 
bolt (fig. 5). The cell and accompanying 
platens are designed to fit "horseshoe" 
fashion about the bolthead for easy in- 
stallation and retrieval. Each U-cell 
was individually calibrated in a stiff 
testing machine to allow the measurement 
of cell pressure. The U-cells can mea- 
sure loads to 30,000 lb with measured ac- 
curacies of ±250 lb. Resin bolt applica- 
tions required that the bolthead be 
threaded to facilitate installation and 
removal. 

The vertical-displacement gauge (fig. 
6) consists of four spring clips used to 
anchor high-strength, stainless steel 
prestretched wire at selected depths in a 
1-3/8-in-diam hole drilled in the mine 
roof. The uppermost spring clip is 
placed in a stable layer to be used as a 
base reference for measured displace- 
ments. For this investigation, a hole 
depth of 7 ft was used. The remaining 
three spring clips are placed at 5 ft, 3 
ft, and 1 ft away from the bolthead. The 
wires from the four spring clips run 
through a 10-in-long tube anchored in 
the collar of the drill hole. The wires 
go through numbered holes in the copper 
cap on the end of the tube and have small 
brass fittings that are used as reference 
points. A loop is made at the end of the 
wires so that a 3-lb weight can be at- 
tached to maintain a constant tension on 
the wire while readings are taken. Read- 
ings are made with a dial indicator 
placed between the cap and the reference 
point on each wire. 

The bearing plates used in the inves- 
tigation were laboratory tested. The 
ASTM standard requires that the plate be 




^^r 



^ 






o 



4 



6 

-j 



Scale, in 

FIGURE 5. -Hydraulic U-cell plates and bladder. 



preloaded to 6,000 fflbf when measuring 
displacements, to within 0.001 in, of the 
axial movement of the bolthead. The load 
is then increased to 15,000 ft'lbf, and 
the axial displacement is read. The max- 
imum permissible deflection between the 
6,000- and 15,000-f f lbf loads is 0.120 




Pipe cap ""' 



Brass reference 
point — . 



^H 



hr 



Holes for wires 
Center hole for gauge 

"- Dial gauge 



ft 



l' \*r- 



3— lb weight 



FIGURE 6.-Vertlcal displacement gauge. 




FIGURE 7. -Roadside Mine test site location. 

in. The plate is then loaded to 20,000 
fflbf, and again the axial displacement 
is measured. The maximum permissible de- 
flection between the 6,000- and 20,000- 
fflbf loads is 0.250 in (7_). 

Six bearing plates were randomly se- 
lected and tested from a purchased lot of 
250. All plates exceeded the ASTM stan- 
dards by a minimum of 15%. 



FIELD INVESTIGATION — ROADSIDE MINE 



The first phase of this investigation 
was conducted in the Roadside Mine. The 
test site was established in a develop- 
ment panel in the No. 2 East Mains, 1st 
East section in one of the deepest areas 
of the mine (fig. 7). The test site 
instrumentation was located approximately 
midway in an 80-ft room and included 44 
compression pads and 6 vertical displace- 
ment gauges. This combination of instru- 
mentation made possible the measurement 
of both the loads on the bearing plates 
and the separations in the immediate 
roof. The test site instrumentation 
was read and evaluated four times in a 
7-month period. 

Bolts used in these test sites were 
standard 0.75-in-diam reinf orcing-steel- 
rod-type, grade 40 (17,600-lb yield 
strength and 30,000-lb tensile strength) 



roof bolts. The bolts were installed in 
1.0-in-diam holes to the specifications 
of the resin manufacturer. The bolt 
characteristics varied from bolt to bolt 
in some cases; all bolt characteristics 
are the minimum documented laboratory 
values. 

Three days after the excavation of the 
opening and the installation of the in- 
strumentation, a distinguishable loading 
pattern was observed (fig. 8). The mini- 
mum and maximum loads measured on the 
bearing plates were 5,700 lb and 29,000 
lb, respectively; the average load on the 
bearing plates in the test area was 
approximately 14,000 lb. The high con- 
centration of loads in the middle third 
of the entry caused the laminated shale 
roof to separate and fall when not con- 
fined by the wire mats installed in 



Roadside Mine 
3 days after installation 




Roadside Mine 
72 days after installation 




Contour irrtervol - 5,000 lb 



Scale, ft 



10 
J 



FIGURE 8 -Roadside Mine load contours 3 days and 72 days after test site Installation. 



conjunction with the bolts. The differ- 
ential sag stations were lost owing to 
their roof deterioration. 

The loading trends observed 72 days 
after installation were similar to those 
recorded at 3 days; however, the loads 
increased by 30% (fig. 9). Loads on the 
bearing plates ranged from 6,100 lb to 



32,000 lb and averaged 18,100 lb. Visual 
examination of the test area revealed 
high degrees of roof spalling, as shown 
in figure 9. 

The measurements recorded 150 days 
after installation were similar to the 
72-day measurements. The minimum and 
maximum loads were 7,400 lb and 32,000 




FIGURE 9.-Roof spelling In Roadside Mine test site. 



lb, respectively. The average load on 
the 44 pressure pads was 18,300 lb, or 
475 psi. These loads generated up to 
41,000 psi of axial stress on the 3/4-in 
bolts, causing them to yield. 

The final measurements were recorded 
205 days after the test site was estab- 
lished. At that time, panel development 
was complete and the pillar retreat line 
was approaching the test site (approxi- 
mately 300 ft away inby). The pillars 
were yielding, by design, resulting in 
high load concentrations that propagated 
toward the test site area. As the pil- 
lars adjacent to the test site yielded, 
the loss of rib coal to sloughing 



resulted in an increase of effective roof 
span to approximately' 30 ft. Figure 
10 shows the final loading pattern. The 
maximum and minimum loads were 7,500 lb 
and 32,000 lb. The average load, mea- 
sured on the bearing plates, was 18,900 
lb. These test results showed, uncondi- 
tionally, that bearing plates were sub- 
jected to high degrees of loading and 
were an important part of the total sup- 
port system. However, because all the 
vertical displacement gauges were lost 
and the borescope holes were closed, the 
roof displacements generated by these 
loads remained unknown. 



FIELD INVESTIGATION — BEAR NO. 3 MINE 



The second phase of the investigation 
was performed in the Bear No. 3 Mine. 
The test site was located in an entry, 
including both a three-way and a four-way 



intersection, under approximately 600 ft 
of overburden (fig. 11). A total of 
5,600 ft^ of roof was instrumented to 
monitor bearing plate loads and roof 



Roadside Mine 
205 days after installation 




Contour interval - 5,000 lb 



10 



Scale, ft 



FIGURE 10. -Final Roadside Mine load contours 205 days after test site Installation. 







WSOS a 






Test area 



ax 



«oc 






aoaaOi 

DDDDa 

a 



FIGURE 11. -Bear No. 3 Mine test site location. 

displacements. To measure the loads on 
the bearing plates, 51 compression pads 
and 26 hydraulic U-cells were placed 
between the mine roof and the bearing 
plates. Additionally, 14 vertical dis- 
placement sag station gauges and 7 fiber- 
optic boreholes were installed to monitor 
roof movements (fig. 12). The test site 
instrumentation was read and evaluated 
seven times in a 10-month period. 

After the test site was instrumented, 
on traditional 20-ft mining cycles, the 
baseline data were recorded. The total 
site was instrumented in 7 days. 



The pillars showed no signs of yielding 
or sloughing. However, a high-angle, 
clay-filled discontinuity, spanning the 
entry at N 60° E, was located 8 ft north 
of the four-way intersection. 

The test site was monitored 50 days 
after installation. The increased load- 
ing on the bearing plates, of 7,000 lb, 
in the four-way intersection was attrib- 
uted to displacements associated with the 
clay-filled discontinuity. The vertical 
displacement gauges in the area recorded 
0.2 to 1.0 in of total displacements. A 
visual observation of the roof area, with 
the aid of a fiber-optic stratascope, 
revealed a minor separation of 0.4 in at 
the 4-ft level. This separation occurred 
between layers of thinly laminated shale. 
Small amounts of loading, approximately 
1,500 lb, developed near the three-way 
intersection. Only small amounts of dis- 
placement were recorded, with no 
apparent visual separations. 

The instrumentation was read and evalu- 
ated at 87 and 128 days after installa- 
tion. The loading pattern and roof dis- 
placements for the 128-day measurements 
are shown in figure 13. The highest de- 
grees of loading were recorded in the 
vicinity of the discontinuity near the 
four-way intersection. Sloughing in the 



10 



Crosscut 21 



B A 

+ b a 

a m a 



Crosscut 20 



+ © 

B B 

SB B 
A A 



a h 

B © 

A -f- A 

B B 

B A 

A A 



B B 
A 



B 
A 



© 
BBS 



© H 

A B 



+ 
B H B 

A A 

© 

BBS 

+ 
A A 

BEE 
+ 
A A B 

+ © + B 

a b b 



LEGEND 

□ Compression pad 

a U-cell 

+ Sag station 

° Fiber-optic 
borehole 



+ 

B B B 

A A 

+ © 
B B 



O) 

£ 

■o 

CM 



10 20 



Scale, ft 



FIGURE "^.-Instrumentation layout In the Bear No. 3 Mine. 

west ribs also contributed additional 
loads on the bearing plates. The average 
load on the bearing plates was approxi- 
mately 5,400 lb. The displacements, 
measured at the 3-ft level, were as high 
as 1.8 in near the four-way intersection 
and 0.9 in in the middle of the three-way 
intersection. Twelve pressure pads were 
recording loads greater than 15,000 lb. 
The loading patterns and roof movements 
recorded at 162 and 211 days were similar 
to those recorded at 128 days. The test 
site results indicated that loads on 
the bearing plates in the four-way 
intersection were increasing at a rate of 



approximately 100 lb per week. The dis- 
placements in the roof remained rel- 
atively consistent with the 128-day 
measurements. The corners of the pillars 
in the three-way intersection were vis- 
ually inspected and observed to be begin- 
ning to yield, creating increased loading 
(fig. 13) in the immediate area. The 
condition of the roof appeared to be sta- 
ble, even with an average of 5,900 lb of 
load being carried by the bearing 
plates. 

Final instrumentation readings were ac- 
quired 311 days after the initial instal- 
lation. The loading patterns and dis- 
placements are shown in figure 13. The 
test site underwent considerable changes 
in loading pattern that were attributed 
to the large degree of observed pillar 
yielding. The effective roof span, due 
to this pillar yielding, had increased by 
16%, or 4 ft, causing an increase in the 
entry's centerline roof displacement. 
High loads were generated along the rib- 
lines in certain areas owing to the ex- 
tended length of unsupported roof in the 
yield zone of the coal pillars. The min- 
imum and maximum loads measured on the 
bearing plates were 600 lb and 26,700 lb, 
respectively. Theoretically, this max- 
imum load would generate approximately 
60,000 psi of pressure on a 3/4-in-diam 
bolt. This pressure exceeded the theo- 
retical yield of the bolt system. The 
final calculated average load on the 
bearing plates was approximately 8,000 
lb, an increase of 38% over previous 
readings. 

The results from the test site in the 
Bear No. 3 Mine indicated, conclusively, 
that bearing plates are subjected to 
loading when installed in conjunction 
with full-column resin-grouted bolts. 
Roof movements and separations, monitored 
with vertical displacement gauges, corre- 
sponded closely with applied bearing 
plate loads. Instrumentation used in the 
two test sites has been shown, through 
past experience, to be both effective and 
reliable. Field data can be closely cor- 
related to the stabilization of the 
entry. 



11 




2,500 





Bear No. 3 


Mine 




128 


days 


after 


installat 


on 



-<%> 




Bear No. 3 Mine 
302 days after installation 




.EGEND 

-5.000— 2,500-lb roof load contour interval 
-1.2 — Roof displacement interval, in 

10 20 



9587 33 



Scole. ft 



FIGURE 13. -Bear No. 3 Mine load and roof displacement contours 128 days and 301 days after test site Installation. 



12 



CONCLUSIONS 



The field data of these two sites indi- 
cate that bearing plates at the heads of 
full-column, resin-grouted bolts can be 
subjected to significant loads. The 
bearing plates not only retain the roof 
material but support large amounts of 
generated load between bolts. The loads 
tend to be closely related to the roof 
movements owing to pillar yielding and 



geologic anomalies. In some instances, 
the loads on the plates were so extreme 
that the ultimate strength of the No. 6 
rebar, grade 40 bolts was approached and 
exceeded. However, overall stability was 
maintained, as evidenced by the retention 
of constant loads measured on the bearing 
plates and negligible vertical displace- 
ment measurement increases. 



REFERENCES 



1. Coates, D. F., and Y. S. Yu. 
Three-Dimensional Stress Distributions 
Around a Cylindrical Hole and Anchor. 
Paper in Proceedings of the Second Con- 
gress of International Society for Rock 
Mechanics (Belgrade, Sept. 21-26, 1970). 
"Jaroslav Cerni" Institute for Develop- 
ment of Water Resources, Belgrade, 1970, 
pp. 175-181. 

2. Haas, C. J., G. B. Clark, and R. N. 
Nitzsche. An Investigation of the Inter- 
action of Rock and Types of Rock Bolts 
for Selected Loading Conditions (contract 
HO 122 110, Univ. MO). BuMines OFR 2-77, 
1974, 342 pp; NTIS PB 267673. 

3. Nitzsche, R. N. , and C. J. Haas. 
Installation Induced Stresses for Grouted 
Roof Bolts. Int. J. Rock Mech. Min. Sci. 
and Geomech. Abstr. , v. 13, No. 1, 1976, 
pp. 17-24. 

4. Khalsa, N. , and L. R. Ladwig 
(eds.). Colorado Coal Analyses 1976— 
1979. CO Geol. Surv. , Inf. Ser. No. 10, 
1981, 246 pp. 



5. Schwochaw, S. D. (comp.). Mineral 
Resources Survey of Mesa County — A Model 
Study. CO Geol. Surv., Resour. Ser. No. 
2, 1978, pp. 35-38. 

6. Osterwald, F. W. , C. R. Dunrud, 
J. B. Bennetti, Jr., and J. 0. Maberry. 
Instrumentation Studies of Earth Trem- 
ors Related to Geology and to Min- 
ing at the Somerset Coal Mine, Col- 
orado. U.S. Geol. Surv. Prof. Paper 
762, 1972, pp. 2-9. 

7. American Society for Testing and 
Materials. Standard Specifications for 
Roof and Rock Bolt Accessories. F-432- 
77 in Annual Book of ASTM Standards: 
Part 10, Bolts, Threaded Bars, Threaded 
Slotted Bars, Bearing and Header Plates, 
and All Types of Washers. Philadelphia, 
PA, 1977, pp. 6-7. 



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